The present invention generally relates to exhaust systems and more particularly to a marine exhaust system which expels exhaust gas to a point outside of the turbuent boundary layer surrounding the moving hull of a boat.
Engine powered boats, such as sport fishing type boats and cruise boats, suffer from a problem resulting from the low pressure area created behind the boat and the associated turbulent boundary layer extending along the sides of the boat. Exhaust gases which are typically expelled either to the rear of the boat or from the side are trapped within the low pressure area behind the boat and may recirculate up to the passenger compartment or cabin of the boat.
Attempts to solve the above problem include the exhaust system described in U.S. Pat. No. 4,019,456 to Harbert. In this system a gas/water mixture is fed to a muffler which first uses the water to attenuate the exhaust noise and then separates the water from the exhaust gas. The water is then expelled from the boat through an exit in the rear of the hull and the gas is passed through a chamber which accelerates it before it is expelled from the boat through an outlet nozzle located in the hullside near the transom. This design suffers from the problem that the nozzle has a fixed outlet area and thus must be designed to be most efficient at the average cruising speed of the engine. When the engine is operated at full throttle speeds, the exhaust back pressure is excessive for the engine. In addition, when the boat is operated at trolling speeds or low engine speeds, the exhaust gases are not expelled far enough from the boat hull to project them outside the effect of the low pressure area behind the boat or the turbulent boundary layer surrounding the hull.
Although one embodiment of the Harbert patent contemplates providing a spring valve 148 which opens to permit passage of additional gas and prevent excessive build up of back pressure in the system, this solution suffers from the problem of the valve being noisy as it flaps open and closed due to the rapid pulsations of the columns of exhaust gases that is typical of reciprocating engines. In addition, the spring valve does not provide controlled expulsion of gases to maximize the velocity of the gases as they leave the nozzle in order to project them outside of the turbulent boundary layer.
Furthermore, since the Harbert muffler design depends on water to attenuate the exhaust noises, the effectiveness of the muffler will vary depending upon the quantity of water within the muffler at any given time. The quantity of water changes with engine speed and thus its effectiveness at attenuating the exhaust noises is limited.
Accordingly, there is a need for a marine exhaust system which will effectively project exhaust gases through or outside of the turbulent boundary layer surrounding a boat hull for any given engine or boat speed.
In addition, there is a need for an exhaust assembly which will effectively separate exhaust gases from water and efficiently attenuate exhaust noises for all operating conditions.